RFC: cross-session restoration and semantic handoff / 跨会话续接与语义交接

[huangruiteng]

After #4330, cross-session continuation still needed an executable design rather than a promise to preserve context. This bilingual RFC amendment distinguishes compatible resume, same-Agent session replacement and cross-Agent takeover, then specifies context preparation, receiver reassessment, execution fencing and automatic return to the original conversation.

It integrates xiaods’ shipped #4094 Stage A with optional Obelisk resume_or_handoff_gap recall: deliberate handoff preserves current obligations and decisions; historical retrieval only fills remaining evidence gaps. Existing work authority remains the writer, and basic continuation does not depend on an index or broader archive permissions. Both related RFCs and the RFC index are updated.

#4330 之后，补齐明确的跨 session 续接方案：区分原会话恢复、同 Agent 换会话、跨 Agent 接管，定义稠密 brief、重新判断、旧执行者隔离及回原入口自动报结论。吸收 xiaods #4094 与 Obelisk RFC，保持当前工作权威唯一；历史召回只补缺口，不成为基础续接的前置。三份 RFC 的中英正文及目录同步。

Delivery plan / 交付计划：新增 A17–A20，覆盖突然退出、旧会话复活、无 provider/越界召回和压缩遗漏；将其接入 M2/M3 及现有前端、飞书、CLI 的验收。按无索引同 Agent 替换 → 跨 Agent 接管与回传 → 可选历史召回推进。

Validation / 验证：

• python examples/docs-governance-smoke.py passed with the project Python 3.13 runtime.
• git diff --check passed; bilingual subsection/acceptance-ID parity and changed-file privacy checks passed.
• Luna/max read-only review completed; strengthened stale-executor negative tests, same-Agent ownership readback, recall-stage admission and the M3 entry-point acceptance sequence.

Product entry points / 产品入口：documentation-only; no runtime, frontend bundle, Lark behavior or provider default changes in this PR. The RFC explicitly requires companion controls/readback/feedback and real packaged-frontend/Lark/CLI acceptance in each implementation slice. These future acceptance cases have not yet been executed. Proposal remains review-required; no self-merge.